Absolutely calibrated, time-resolved measurements of soft x rays using transmission grating spectrometers at the Nike Laser Facility

Accurate simulation of pellet implosions for direct drive inertial confinem ent fusion requires benchmarking the codes with experimental data. The Nava l Research Laboratory (NRL) has begun to measure the absolute intensity of radiation from laser irradiated targets to provide critical information for the radiatively preheated pellet designs developed by the Nike laser group . Two main diagnostics for this effort are two spectrometers incorporating three detection systems. While both spectrometers use 2500 lines/mm transmi ssion gratings, one instrument is coupled to a soft x-ray streak camera and the other is coupled to both an absolutely calibrated Si photodiode array and a charge coupled device (CCD) camera. Absolute calibration of spectrome ter components has been undertaken at the National Synchrotron Light Source at Brookhaven National Laboratories. Currently, the system has been used t o measure the spatially integrated soft x-ray flux as a function of target material, laser power, and laser spot size. A comparison between measured a nd calculated flux for Au and CH targets shows reasonable agreement to one- dimensional modeling for two laser power densities. (C) 2001 American Insti tute of Physics.